Initial catabolic steps of isoleucine, the R-pathway and the origin of alloisoleucine

The initial catabolic steps of isoleucine by mammals has been misunderstood and misapprehended in the scientific literature for many years. The suggestion that the interconversion of isoleucine and alloisoleucine occurs through the keto–enol racemization of their respective transaminated α-keto acids was first tentatively advanced by Alton Meister in the early 1950s, and accepted without hard confirming evidence by many authors. It will be shown in this brief review that isoleucine is converted to alloisoleucine with conservation of a 15N label denying the intermediacy of the α-keto acids, and that alloisoleucine arises as an unavoidable consequence of isoleucine transamination.